2 Answers

Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color.

Basically using 24 as the pure base, each lesser 'k' (22, 18, 14 and 10) would be subtracted from the base (24) although it may be described as percentages such as 25% alloy 75% gold, etc. But for simplicity you'd compare the 22, 18, 14 and 10 from 24 (pure).

Gold dust is a quite uncertain commodity: assay will reveal what is in it, in what percentages, and whether or not it is a commercially viable sample. Unassayed gold dust might be worth only $150.00/oz, considering the level of contaminants and pyrites that may be in it.